174 RICHARD ASSHETON, M.A. 



The relation of these two parts is shown diagrammatically in fig. 12, 

 which represents a rather earlier stage (tadpole 9 mm.). 



The further discussion as to the fate of the nervous and epidermic 

 layers of epiblast I defer to a paper on the development of the central 

 nervous system of the frog, on which I am at present engaged. 



I think, however, that there is no doubt that the spongioblastic 

 elements are all derived from the epidermic layer, and the neuroblasts 

 from the nervous layer of the primitive epiblast. 



To return to the section. Next to the pigment layer are seen the 

 developing rods and cones. The former are more developed than the 

 latter, and already their processes, which project into the cavity of the 

 optic vesicle, show the division into inner and outer limbs. 



Connected with the outer molecular layer are certain neuroblasts of 

 the inner nuclear layer (X). 



Among the cells of the outer nuclear layer are some with long, 

 broad prolongations (FR) the radial or Miillerian fibres of the adult. 



These fibres are not easily traced to the "outer limiting membrane" 

 in the centre of cup, but may be more easily seen towards the rims, 

 where the rods and cones are not so crowded (FR'). Compare these 

 with the cells marked SP in fig. 12. 



Certain neuroblasts (NI) of the inner nuclear layer seem to send 

 processes into the inner molecular layer, but my observations do not 

 enable me to say whether they break up into fibrils, or whether they 

 pass through and on to the brain. 



Within the inner molecular layer comes a double row of cells, the 

 ganglionic layer. In many cases these cells are produced into short 

 deeply staining processes towards the inner molecular layer, and break 

 up at once into fine fibrils (G.). 



The growth of these processes seems to be the prime cause of the 

 appearance of the inner molecular layer. 

 These processes just described appear to be a later development than 

 the process from the opposite pole, which forms a fibre of the optic 

 nerve. 



These are seen at N, near the rim of the cup, as thick black pro- 

 longations, while the other pole is still round, and shows as yet no 

 signs of the processes described above, which form, at any rate in part, 

 the inner molecular layer. 



